Basic Operating Procedures for the Start Up and Shut Down of a Typical Shell and Tube Heat Exchanger and Operator Responsibilities Associated with Operating

In this blog , we will learn some basic procedures for the start up and shut down of a typical shell and tube heat exchanger.

We will also go through some operator responsibilities associated with operating a shell and tube heat exchanger.

 Many industrial processes must heat or cool fluids to produce products. Heating and cooling are often accomplished by transferring heat between fluids.

Heat transfer between fluids occurs in heat exchangers. There are many types of heat exchangers, but one of the most common types is a shell and tube heat exchanger. Shell and tube heat exchangers can be used for a variety of processes and operating procedures may vary with each heat exchanger.

This blog post is intended to be a guide to the steps that often have to be taken in the startup of a shell and tube heat exchanger after it has been shut down for maintenance.

But remember, always follow your facility's operating procedures when you're starting up any heat exchanger.

The unit we'll be using is part of a mixing process and is used to cool the process liquid.

The heat exchanger has cooling water inlet

and cooling water outlet lines,

process liquid inlet

and process liquid outlet lines,

a series of isolation valves

a vent valve

and drain valves.

Before starting up the heat exchanger, the operator inspects the unit to see if it's ready. He checks the valves to make sure that all of the isolation vent and drain valves are shut. After he completes the initial inspection, he establishes the cooling water supply to the unit. He does this by calling the control room operator and having him open the shell side vent. Then he partially opens the shell side water inlet valve to slowly fill the shell side. Then he calls the control room to have someone there start the cooling water pump. When the cooling water pump is started, cooling water will fill the shell side of the heat exchanger.

Any air that is trapped on the shell side escapes through the open vent valve. As the shell side of the heat exchanger fills, the operator listens for air escaping.

 hen the shell side is completely filled, he calls the control room and has them shut the vent valve. Then the operator opens the shell side inlet valve the rest of the way. At that point, he informs the control room that the shell side is lined up and that they can establish the proper flow rate through the shell.

 On the heat exchanger shown in this example, there is no isolation valve on the outlet of the shell side.

Some shell and tube heat exchangers have an isolation valve on this outlet. With those heat exchangers, the isolation valve must be open to complete the flow path through the unit.

The next step is to line up the tube side of the heat exchanger.

First, the operator opens the tube side vent valve. Then he partially opens the tube side inlet valve to allow the tube side to fill with process liquid and to remove any air or other gases that might be trapped on the tube side of the unit. When the tube side is filled, the operator closes the vent valve and opens the inlet valve the rest of the way.

Then he opens the outlet valve. At this point, the startup is complete and the heat exchanger is in operation.

After the operator reports to the control room that the unit is in operation, the control room establishes flow through the unit.

Shell and tube heat exchangers are commonly used in a variety of processes and each may have a different operating procedure.

But remember, always follow your facility's operating procedures when shutting down any heat exchanger.

During shutdown, the side of the heat exchanger with the hotter fluid is usually shut down first. This helps to prevent the heat exchanger from being overheated and damaged.

 

Let's watch an operator as he takes a heat exchanger out of service.

In this example, the heat exchanger, which cools a product from a reactor, is being shut down for maintenance.

The tube side fluid is the hotter fluid. To shut down the heat exchanger, the operator closes the tube side inlet valve first, and then the tube side outlet valve. When the tube side has cooled, the operator opens the tube side drain and vent valves. This allows air to enter the tube side and drains the process fluid.

The operator then shuts the shell side inlet valve.

On the some heat exchanger the shell side has no isolation valve on its outlet. Some shell and tube heat exchangers have an isolation valve on this outlet. With those heat exchangers, the isolation valve must be shut to completely isolate the heat exchanger. After the isolation valve is shut, the operator opens the cooling water drain and vent valves.

Once all of the fluid is drained from the heat exchanger, the operator closes the vent and drain valves to complete the shutdown.

 

Now, if the heat exchanger that's being shut down handles flammable liquids, it may have to be purged to reduce the possibility of a fire or an explosion caused by flammable vapors.

Purging means forcing the process fluids out of the heat exchanger by a substance that won't react with the process fluid. Very often, steam or nitrogen is used.

When a heat exchanger is in operation, operators must routinely check the unit to ensure that it's operating properly.

This may include checking temperature and pressure instruments to make sure that their readings are within normal operating ranges, as well as checking the condition of the heat exchanger itself.

By checking a heat exchanger's temperature instruments, an operator can tell how the temperatures of the fluids change as they pass through the unit. These values can also be used to determine the difference in temperature, or delta T, for each fluid. The delta T can be used to see if the unit is operating properly.

For example, if the delta T across the tube side of a heat exchanger is supposed to be 10 degrees, but it is only 5 degrees, it could be an indication that the tubes in the unit are becoming fouled, or that one or both of the flow rates are not correct for proper operation.

In any event, the cause of the problem should be investigated, and supervisory personnel should be informed.

Another way that temperatures can be checked is on a temperature recorder. The recorder plots temperature values on a chart, which allows an operator to see if a trend is developing.

Additional information about a heat exchanger can be obtained by checking the unit's pressure instruments.

By reading the pressure instruments, an operator can often detect problems with flow through the unit. Any time there's flow through a heat exchanger, there will be a specific drop in pressure across the unit. This pressure drop is often referred to as differential pressure, or delta P.

Any change in differential pressure could be an indication of a problem. As the tubes become blocked or fouled, the differential pressure will increase above the normal value.

Once again, the cause of the problem should be investigated, and supervisory personnel should be informed.

On many units, temperature and flow are controlled by automatic systems. These systems may provide indications locally and in a control room. The indications in the control room can be compared to the indications on instruments located at the heat exchanger to verify that the heat exchanger is operating properly.

If a problem is detected, it could be the result of the control valves not operating properly, or valves being out of position. Or it could be an indication that other equipment associated with the heat exchanger is not operating properly.

For example, a low inlet pressure could indicate a problem with a pump that supplies the heat exchanger.

Besides checking instrument readings, an operator should also check for leaks and for damaged or missing insulation.

Problems such as these could affect the operation of the heat exchanger and pose hazards to personnel working in the area.

In this topic, we looked at some basic procedures for the start up and shut down of a typical shell and tube heat exchanger. We also looked at some operator responsibilities associated with operating a shell and tube heat exchanger.

 

Now let's summaries what we learned.

When the cooling water pump is started, cooling water will fill the shell side of the heat exchanger.

Any air that is trapped on the shell side escapes through the open vent valve. As the shell side of the heat exchanger fills, the operator listens for air escaping.

When the shell side is completely filled, he calls the control room and has them shut the vent valve.

Then the operator opens the shell side inlet valve the rest of the way.

At that point, he informs the control room that the shell side is lined up and that they can establish the proper flow rate through the shell.

To shut down the heat exchanger, the operator closes the tube side inlet valve first and then the tube side outlet valve.

When the tube side has cooled, the operator opens the tube side drain and vent valves. This allows air to enter the tube side and drains the process fluid.

As the tubes become blocked or fouled, the differential pressure will increase above the normal value.

Once again, the cause of the problem should be investigated and supervisory personnel should be informed.

 

“Thanks for Reading !”